1. Field of the Invention
The present invention relates to a coupled FBAR filter usable for balancing; and a ladder-type filter, a duplexer and a communication device including such a coupled FBAR filter.
2. Description of the Background Art
Conventionally, filters mounted on wireless communication devices such as cellular phones include dielectric filters, laminated filters and acoustic filters. Known acoustic filters include a monolithic crystal filter (MCF) using a plurality of modes of a bulk wave and a surface acoustic wave filter (SAW filter). Recently, filters are required to have a smaller size, to provide a higher performance and to be usable at a higher frequency. As a device for fulfilling these requirements, a film bulk acoustic resonator filter (FBAR filter) using a bulk wave of a piezoelectric thin film has been developed. As an FBAR filter, a coupled FBAR filter realized by multiplexing a plurality of modes is proposed.
Recently, a balancing performance of semiconductor components such as ICs has been enhanced for the purpose of improving the signal-to-noise characteristic, and an FBAR used at an RF stage and a filter including such the FBAR are now desired to provide a better balancing performance.
Also recently, it has been desired to control the impedance of an FBAR filter in order to match the impedance with ICs located at stages before and after the FBAR filter.
A conventional balanced type MCF using a plurality of modes (Japanese Laid-Open Patent Publication No. 2001-53580) will be described. FIG. 12 shows a structure of the conventional balanced type MCF.
The MCF includes three electrode pairs, i.e., the electrode pair 72/76, the electrode pair 73/74, and the electrode pair 75/77. The electrodes of each pair face each other with a crystal substrate 71 (piezoelectric substrate) interposed therebetween. The electrode 72 on the left is an input electrode, and the electrode 73 on the right is an output electrode. The input electrode 72 and the assisting electrode 76 facing the input electrode 72 are respectively connected to input balanced terminals 711 and 713, and the output electrode 73 and the assisting electrode 74 facing the output electrode 73 are respectively connected to output balanced terminals 714 and 712. The MCF excites a first-order thickness vibration and a third-order thickness vibration and thus realizes a high coupling degree of the plurality of modes. Thus, a balanced type filter usable over a wide band and having a high attenuation between the input balanced terminals and the output balance terminals is realized.
A method for controlling the impedance of a conventional balanced type filter (Japanese Laid-Open Patent Publication No. 2003-92526) will be described. FIG. 13 shows a structure for controlling the impedance of balanced terminals in a conventional balanced type SAW filter.
The SAW filter has a pattern of electrodes crossing in a periodical strip-line manner on a piezoelectric substrate 601. Owing to this pattern of electrodes, an acoustic surface wave can be excited. On the piezoelectric substrate 601, a longitudinal-mode SAW filter including input IDT electrodes 602a and 602b, reflector electrodes 603a and 603b, and an output IDT electrode 604 is provided. Upper electrodes of the input IDT electrodes 602a and 602b are connected to an input terminal S, and lower electrodes of the input IDT electrodes 602a and 602b are grounded. The output IDT electrode 604 is divided into three, i.e., first, second and third divided IDT electrodes 604a, 604b and 604c. The output IDT electrode 604 is formed of a group of the three IDT electrodes 604a, 604b and 604c connected to each other. The first through third divided IDT electrodes 604a, 604b and 604c are located to have the same phase. Upper electrodes 605a and 605b of the first and second divided IDT electrodes 604a and 604b are electrically connected to each other and are also connected to one balanced output terminal T1. Lower electrodes 606b and 606c of the second and third divided IDT electrodes 604b and 604c are electrically connected to each other and are also connected to the other balanced output terminal T2.
FIG. 14 shows a capacitance equivalent circuit diagram between the output balanced terminals of the balanced type SAW filter shown in FIG. 13. Capacitances Ca through Cc are respectively capacitances of the first through third divided IDT electrodes 604a through 604c, and a synthesized capacitance of the capacitances Ca through Cc is a total capacitance Cout of the output IDT electrode 604. By varying the logarithm of the electrode fingers included in the output IDT electrode 604, the value of the total capacitance Cout can be controlled. Since the impedance of the SAW filter dominantly depends on the capacitances of the IDT electrodes, the impedance between the output balanced terminals can be controlled by varying the logarithm of the first through third divided IDT electrodes 604a through 604c. 
Hereinafter, a method for controlling the impedance of a conventional coupled FBAR filter having balanced/unbalanced conversion type input and output terminals will be described.
FIG. 15A is an isometric view showing a structure of a conventional balanced/unbalanced type coupled FBAR filter 900. FIG. 15B is a cross-sectional view of the conventional balanced/unbalanced type coupled FBAR filter 900 shown in FIG. 15A taken along line E-E.
The conventional balanced/unbalanced type coupled FBAR filter 900 includes first and second lower electrodes 92 and 94, a piezoelectric thin film 91, and first and second upper electrodes 93 and 95, which are provided on a substrate 90. The first lower electrode 92, the first upper electrode 93, and the piezoelectric thin film 91 interposed between these electrodes form a first vibration section. The first upper electrode 93 is used as an input terminal, and the first lower electrode 92 is used as a GND terminal. The second lower electrode 94, the second upper electrode 95, and the piezoelectric thin film 91 interposed between these electrodes form a second vibration section. The second upper electrode 95 is used as an output balanced terminal (+), and the second lower electrode 94 is used as an output balanced terminal (−). The substrate 90 has a cavity 96 formed therein, which is capable of commonly guaranteeing vibrations of the first and second vibration sections and coupling the vibrations. Owing to this, for example, the second vibration section on the output side excites a mechanical vibration by the mechanical vibration excited in the first vibration section on the input side, and second vibration section on the output side further converts the excited mechanical vibration into an electric signal and outputs the electric signal.
In general, the vibration excited in a thickness direction in the second vibration section on the output side is at λ/2. Where the resonant frequency in the second vibration section is fr and the average sonic speed in the thickness direction is V, the wavelength λ is represented by V/fr. Accordingly, in the case where the second vibration section is designed to have such a thickness as to obtain the resonant frequency fr, an electric signal which is output from a top surface of the piezoelectric thin film 91 and an electric signal which is output from a bottom surface of the piezoelectric thin film 91 at the resonant frequency fr would ideally have a phase difference of 180 degrees and an amplitude difference of 180 degrees. Therefore, a single phase electric signal which is input to an input terminal IN1 is converted into differential electric signals by the balanced/unbalanced type coupled FBAR filter 900 and is output from output terminals OUT1(+) and OUT2(−).
The second lower electrode 94 and the second upper electrode 95 are respectively shorter in an x direction than the first lower electrode 92 and the first upper electrode 93. Owing to this, the electrode area size is decreased, and thus an effect of reducing the damping capacitances which form parallel capacitances of the equivalent circuit is provided. Therefore, the impedance at the output balanced terminals can be increased.
FIG. 16A is an isometric view showing a structure of another conventional balanced/unbalanced type coupled FBAR filter 800. FIG. 16B is a cross-sectional view of the conventional balanced/unbalanced type coupled FBAR filter 800 shown in FIG. 16A taken along line F-F. The structure of the balanced/unbalanced type coupled FBAR filter 800 is different from that of the balanced/unbalanced type coupled FBAR filter 900 only in the shape of the second upper electrode and the second lower electrode. The principle of controlling the impedance is the same and will not be repeated here.
FIG. 17 shows ideal vibration mode distributions of the conventional balanced/unbalanced type coupled FBAR filter 900 shown in FIG. 15A and FIG. 15B. An integral value of the vibration mode distribution curves is generally equivalent to the amount of charge generated in the piezoelectric thin film 91. By efficiently using the generated charge for the vibrations in the two vibration sections, the coupling coefficient of each vibration section can be increased, and smaller-loss and wider-band characteristics can be realized.
However, in the balanced/unbalanced type coupled FBAR filter 900, the area size of the second lower electrode 94 and the second upper electrode 95 is smaller than the area size of the first lower electrode 92 and the first upper electrode 93. Therefore, the amount of charge generated in the second vibration section is smaller. For this reason, the charge generated in the first vibration section cannot be efficiently used in the second vibration section, which increases the loss and narrows the resonant frequency band. The balanced/unbalanced type coupled FBAR filter 900 can significantly convert the impedance, but has a problem that the loss is increased and the resonant frequency band is narrowed.
FIG. 18 shows ideal vibration mode distributions of the conventional balanced/unbalanced type coupled FBAR filter 800 shown in FIG. 16A and FIG. 16B. In the balanced/unbalanced type coupled FBAR filter 800, there is a discontinuous surface between the first vibration section and the second vibration section with respect to the vibration propagating in the x direction. On the discontinuous surface, an unnecessary mode is generated. In addition, the ratio in an area above the cavity occupied by a non-electrode portion is higher than that in the conventional balanced/unbalanced type coupled FBAR filter 900. In other words, the electrode area size of the second vibration section is smaller than the electrode area size of the first vibration section. Therefore, the charge generated in the first vibration section cannot be efficiently used in the second vibration section. This increases the loss, narrows the resonant frequency band, and thus deteriorates the characteristics. The balanced/unbalanced type coupled FBAR filter 800 can significantly convert the impedance, but has a problem that the loss is increased, the unnecessary mode is generated, and the resonant frequency band is narrowed.